Quiz
Cross-word Puzzle

Accuracy - The degree to which an indicated value matches the actual value of a measured variable. In instrumentation, the ratio of the error to the full-scale output, usually measured as + ___ percent of full-scale output.

Actuator - A part of the final control element that translates the control signal into action of the final control device in the process. A device that actuates a final control element such as a control valve.

Analog - Any continuously variable physical quantity. Voltage, current, and resistance are all analog quantities, and can be expressed as continuous waveforms.

Analog-to-digital (A/D) converter - A device that converts analog information (voltage, frequency, resistance, etc.) to approximate corresponding digital information.

Bourdon tube - A pressure sensing element consisting of a twisted tube of noncircular cross section closed at one end. When a process stream is routed to the open end of the tube, any increase in the pressure will cause the tube to unwind.

Bridge circuit - An electronic network in which an input voltage is applied across two parallel elements and an output voltage -- to an indicating device -- is taken across two intermediate points on the parallel elements. A bridge circuit is used to determine an unknown resistance. Typically used with strain gages, and RTDs.

Buoyancy - The tendency of a fluid to lift any body submerged in the fluid. The amount of force applied to the body equals the product of the fluid density and the volume of the body displaced.

Bus - A group of wires or conductors considered to be a single entity. A common connector circuit, usually multiwire, for transfer of power, data, etc. between several modules on the bus.

Capacitance - The ability of a condensor to store a charge before the terminals reach a potential difference of one volt.

Capacitor - A device used to store an electric charge.

Capillary - A tube with a very small diameter .

Cascade control - A type control action in which the output of one controller is the setpoint of another controller. The purpose of this type of control is to reduce the effects of disturbances on the primary control parameter and to improve performance of the primary loop.

Coefficient of linear expansion - A constant that defines the degree of expansion that will take place per degree Centigrade of temperature increase. For example, the coefficient for aluminium is 25 x 10^-6 / degC.

Controlled variable - The variable that the control system attempts to keep at the set point value. The set point may be constant or variable.

Control valve - Any valve that controls pressure, flow rate, or flow direction in a fluid or gas-filled system. The control valve is a final control element that adjusts the size of the flow passage as directed by a signal from the controller to modify the rate of flow of the fluid.

Controller - A device that operates automatically to regulate a controlled variable. An automatic controller varies its output signal in response to changes in the measured variable. The output of a manual controller can be varied only by manual adjustment.

Data highway - A communications link between separate stations. It eliminates the need for separately wired datalinks. Each station on a highway can function independently.

Dead band - The range through which an input can be varied without causing a measurable response. Dead band is usually expressed in percent of span.

Derivative control action (rate) - A type of control action in which the output is proportional to the rate of change of the input.

Diaphragm - A sensing element consisting of a thin, usually circular disc that is deformed by a difference in pressure. The device is usually supported around the edges, and the center of the disc moves in a direction perpendicular to the plane of the disc.

Digital - The use of binary digits (1's and 0's) to represent continuous values or discrete states.

Direct Digital Control (DDC) - A computer control technique that sets the final control element's position directly by computer output. The computer carries out functions normally performed by a controller.

Distributed control system (DCS) - A system of dividing plant or process control into several areas of responsibility, each managed by its own controller, with the whole system connected to form a single entity, usually by means of communication buses.

Electronic transmission - A transmission system that uses the flow of electric current (usually 4 - 20 milliamperes) to transmit output or input signals.

Feedback - The process signal used in control as a measure of response to control action.

Feedback control loop - A signal path which includes a forward path, a feedback path, and a summing point.

Feedforward control - A method of control in which information concerning one or more conditions that can disturb the controlled variable is converted, outside of any feedback loop, into corrective action to minimize deviations of the controlled variable.

Governor - A device that automatically regulates the speed or power of another device (typically an internal combustion or steam engine).

Heat sink - Any device used primarily to absorb heat from, and thereby protect, another device from damage due to excessive heat.

Hydraulic - Any device or operation that uses pressure or flow of oil, water, or other liquid of low viscosity.

Hydrostatic head - The pressure created by the height of a liquid above a given point.

Integral action - A type of controller function wherein the ouput (control) signal or action is a time integral of the input (sensor) signal.

Magnetic flowmeter - A device that uses magnetic effects to measure the flow rate of conductive liquids. This type of flowmeter presents very little obstruction to the flow of the liquid being measured.

Manometer - A gage for measuring pressure or pressure differences. The U-tube manometer consists of two legs, each containing a liquid of known specific gravity.

Manipulated variable - The quantity or condition that is varied as a function of the actuating error signal in order to change the value of the directly controlled variable.

Measured variable - A quantity, property, or condition that is measured. Also referred to as the measurand. The part of the process that is measured.

Microprocessor - A very large-scale integrated circuit that has all the functions of a computer except for memory and input/output systems

Noise - An unwanted component of a signal. Meaningless signals similar to radio static. Some noise interferes with the correct output of a signal.

Offset - A sustained deviation of the controlled variable from setpoint. Offset is caused by load changes and is inherent in the use of proportional controllers that do not incorporate reset action.

Pascal's Law - Whenever an external pressure is applied to any confined fluid at rest, the pressure is increased at every point in the fluid by the amount of the external pressure. A practical application of this law is a hydraulic press.

Piezoelectric effect - The generation of an electric potential when pressure is applied to certain crystallized materials.

Pneumatic - A device that converts the enrgy of a compressed fluid (typically air) into motion.

Positioner - A device that can be used to modify the action of a valve, extend the stroke/controller signal, increase the pressure to the valve actuator, or modify the control valve flow characteristics.

Precision - The degree of reproducibility among several independent measurements.

Process - A physical or chemical change of matter, or conversion of energy.

Process control - The regulation or manipulation of variables influencing a process in such a way to obtain the desired quantity and quality of product in an efficient manner.

Programmable logic controller (PLC) - A microprocessor-based controller that has multiple inputs and outputs, and that contains an alterable program. A PLC has a user programmable memory for storage of instructions to implement specific functions such as input/output control, timing, counting and control.

Proportional band - The change in input required to produce a full range change in output due to proportional control action. This value is the reciprocal of the proportional gain.

Proportional control action - Corrective action which is proportional to the error. The change of the manipulated variable is equal to the gain of the proportional controller multiplied by the error (activating signal).

Proportional gain - The ratio of the change in output due proportional control action to the change in input

Proportional-plus-integral control action - Control action in which the output is proportional to the linear combination of the input and the time integral of the input.

Rangeability - Describes the relationship between range and the minimum quantity that can be measured. Installed rangeability may be defined as the ratio of maximum to minimum flow that within which limits the deviation from a desired flow characteristic does not exceed some stated limit.

Ratio control - A method of control in which a predetermined ratio between two or more variables is maintained. Commonly used when mixing two or more materials.

Resistance temperature detector (RTD) - A component of a resistance thermometer consisting of a material whose electrical resistance is a known function of temperature.

Semiconductor - Materials (silicon, germanium, etc.) used as the basis for sensing elements whose resistances to electric current fall between that of conductors and insulators.

Sensitivity - The ratio of the change in output to the change in input that causes it after the steady-state has been reached. Also, the smallest changed in actual value of a measured quantity that will produce an observable change in an instrument's indicated output.

Sensor - The part of the loop or instrument that senses the value of a process variable and sends a corresponding predetermined output. The device may be part of another device (a transmitter, for example) or may be separate. It is also known as a "detector" or "primary element".

Smart device - Any microprocessor based device that can be programmed; has a memory; can perform calculations; perform self-diagnostics; reports faults; and can be communicated with from a remote location. A smart transmitter can be programmed with the type of sensor, range, can linearize the signal, and can correct for errors due to ambient temperature.

Smart transmitter - A microprocessor-based transmitter that can be programmed; has a memory; can perform calculations; perform self-diagnostics; reports faults; and can be communicated with from a remote location.

Setpoint - An input variable that sets the desired value of the controlled variable. The set point may set manually or automatically. Also, the position at which the control point setting mechanism is set. This is the same as the desired value of the controlled.

Specific gravity (S.G.) - A unitless number that is the ratio of the density of a material to the density of a standard. Water (at 60 F) is the standard for liquids. Air (at standard conditions) is the standard for gases.

Supervisory control - A method of control in which a computer generates setpoints for controllers.

Thermistor - A temperature transducer constructed of semiconductor material and for which the temperature is converted into a resistance.

Thermocouple - A temperature measuring instrument that develops an electric voltage due to the joining together of two dissimilar wires. The voltage generated is proportional to temperature when the junction of the wires are heated.

Time constant - The length of time required for the output of a transducer to rise to 63.2 percent of its final value as a result of a step change in the measurand.

Torque - A rotary force, such as that applied by a rotating shaft at any point on its axis of rotation.

Transducer - A device that receives information in one form and converts it to another form. The transducer may also serve as a signal converter.

Transmitter - A transducer that responds to a measured variable by means of a sensing element and converts it to a standardized transmission signal that is a function of the measured variable. The sensor may or may not be part of the transmitter.

Trial and error tuning - Trial

Valve - A device used for the control of fluid flow. Valves can interrupt flow, regulate flow, or divert flow.

Variable - A physical quantity or property that can be measured. Also known as the measurand.

Ziegler-Nichols method - A method used to determine the optimum controller settings when tuning a process control loop. It is based on finding the proportional gain that causes instability in a closed loop.